US8963978B2 - Exposure apparatus with correction for variations in sensitivity and image forming apparatus using the same - Google Patents

Exposure apparatus with correction for variations in sensitivity and image forming apparatus using the same Download PDF

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Publication number
US8963978B2
US8963978B2 US13/442,596 US201213442596A US8963978B2 US 8963978 B2 US8963978 B2 US 8963978B2 US 201213442596 A US201213442596 A US 201213442596A US 8963978 B2 US8963978 B2 US 8963978B2
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light source
light
photosensitive member
laser
image
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US20120268723A1 (en
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Yuichi Seki
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Canon Inc
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Canon Inc
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    • G—PHYSICS
    • G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
    • G03G—ELECTROGRAPHY; ELECTROPHOTOGRAPHY; MAGNETOGRAPHY
    • G03G15/00—Apparatus for electrographic processes using a charge pattern
    • G03G15/04—Apparatus for electrographic processes using a charge pattern for exposing, i.e. imagewise exposure by optically projecting the original image on a photoconductive recording material
    • G03G15/043—Apparatus for electrographic processes using a charge pattern for exposing, i.e. imagewise exposure by optically projecting the original image on a photoconductive recording material with means for controlling illumination or exposure
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/04—Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa
    • H04N1/113—Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using oscillating or rotating mirrors
    • H04N1/1135—Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using oscillating or rotating mirrors for the main-scan only
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/04—Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa
    • H04N1/12—Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using the sheet-feed movement or the medium-advance or the drum-rotation movement as the slow scanning component, e.g. arrangements for the main-scanning
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N1/00—Scanning, transmission or reproduction of documents or the like, e.g. facsimile transmission; Details thereof
    • H04N1/04—Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa
    • H04N1/19—Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using multi-element arrays
    • H04N1/191—Scanning arrangements, i.e. arrangements for the displacement of active reading or reproducing elements relative to the original or reproducing medium, or vice versa using multi-element arrays the array comprising a one-dimensional array, or a combination of one-dimensional arrays, or a substantially one-dimensional array, e.g. an array of staggered elements
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N2201/00—Indexing scheme relating to scanning, transmission or reproduction of documents or the like, and to details thereof
    • H04N2201/0077—Types of the still picture apparatus
    • H04N2201/0082—Image hardcopy reproducer

Definitions

  • the present invention relates to an exposure apparatus and an image forming apparatus using the same.
  • An electrophotographic image forming apparatus generally exposes the surface of a photosensitive member to a light beam (laser beam) output from a light source (laser light source) provided in an exposure apparatus such as an optical scanning apparatus, thereby forming an electrostatic latent image on the surface of the photosensitive member.
  • the photosensitive member on which an electrostatic latent image is formed may have a sensitivity which varies in each individual position (region), that is, unevenness of sensitivity on its surface. This unevenness of sensitivity may occur due to the difficulty in maintaining the thickness of a photosensitive layer of a photosensitive member constant in a process of manufacturing the photosensitive member.
  • the surface potential of the photosensitive member varies in each individual region even if the surface of the photosensitive member is charged by a charging device under the same conditions and exposed by an exposure apparatus under the same conditions.
  • potential unevenness error
  • density unevenness may occur in the formed image upon development of the electrostatic latent image using a developing material such as toner.
  • the power of light from the laser light source is controlled using a correction value corresponding to unevenness of sensitivity of a photosensitive member so as to uniform the surface potential (each of the dark portion potential and light portion potential) of an electrostatic latent image formed on the photosensitive member. More specifically, a correction value for the sensitivity in each region on the photosensitive member is determined from sensitivity data representing this sensitivity, and a drive current to be applied to the laser light source is corrected in accordance with the determined correction value.
  • a drive current applied to the laser light source is corrected in accordance with unevenness of sensitivity of the photosensitive member to correct, in turn, an error of the surface potential of the photosensitive member.
  • the laser light source irradiates the photosensitive member with a laser beam in light power corresponding to image information in accordance with light emitting characteristics determined depending on the drive current controlled by such light power control.
  • the drive current applied to the laser light source is changed by such light power control, the light emitting response of the laser light source changes. In this case, due to this change in light emitting response, the light emitting characteristics of the laser light source shift from ideal light emitting characteristics determined depending on the drive current.
  • the drive current is modulated (by, for example, pulse width modulation) in accordance with image information so that the laser light source emits light in power corresponding to the image information
  • an error may occur between the actual power of light and the ideal power of light determined depending on the drive current.
  • the photosensitive member is irradiated with a laser beam corresponding to the image information, the above-mentioned error resulting from unevenness of sensitivity of the photosensitive member remains in the surface potential of the photosensitive member, so the image quality may degrade.
  • the present invention has been made in consideration of the above-mentioned problem, and provides a technique to reduce a potential error that occurs on the surface of a photosensitive member due to a change in light emitting response of a light source in an exposure apparatus.
  • an exposure apparatus comprising: a light source configured to emit a light beam exposing a photosensitive member; and a control unit configured to cause the light source to execute a first exposure operation exposing the photosensitive member with the light beam emitted from the light source in accordance with a drive current based on image information, and cause the light source to execute a second exposure operation exposing the photosensitive member with the light beam emitted from the light source in accordance with a corrected current based on a position on a surface of the photosensitive member.
  • an image forming apparatus comprising: a photosensitive member; a charging unit configured to charge a surface of the photosensitive member; an exposure apparatus configured to expose the surface of the photosensitive member with a light beam to form an electrostatic latent image on the surface of the photosensitive member; and a developing unit configured to develop the electrostatic latent image formed on the surface of the photosensitive member using a developing material to form, on the surface of the photosensitive member, an image to be transferred onto a printing material
  • the exposure apparatus comprising a light source configured to emit a light beam exposing the photosensitive member, and a control unit configured to cause the light source to execute a first exposure operation exposing the photosensitive member with the light beam emitted from the light source in accordance with a drive current based on image information, and cause the light source to execute a second exposure operation exposing the photosensitive member with the light beam emitted from the light source in accordance with a corrected current based on a position on a surface of the photosensitive member.
  • the present invention it is possible to provide a technique to reduce a potential error that occurs on the surface of a photosensitive member due to a change in light emitting response of a laser light source in an exposure apparatus.
  • FIG. 1 is a block diagram showing the configuration of an image forming apparatus 1 according to an embodiment of the present invention
  • FIG. 2 is a view showing the configuration of an optical scanning apparatus (exposure apparatus) 2 according to the embodiment of the present invention
  • FIG. 3 is a flowchart showing the procedure of an image forming operation by the image forming apparatus 1 according to the embodiment of the present invention
  • FIG. 4 is a circuit diagram showing the configuration of a laser driving device 12 according to the embodiment of the present invention.
  • FIG. 5 is a timing chart showing the timings of the operations of the laser driving device 12 according to the embodiment of the present invention.
  • FIGS. 6A and 6B are views showing the orders of use of main beams and sub-beams which irradiate each main scanning line in respective exposure schemes according to the embodiment of the present invention
  • FIG. 7 shows graphs of the light emitting characteristics of a semiconductor laser 11 and the latent image characteristics on a photosensitive drum 4 when exposure is done as background exposure according to the embodiment of the present invention
  • FIG. 8 shows graphs of the light emitting characteristics of the semiconductor laser 11 and the latent image characteristics on the photosensitive drum 4 when exposure is done as image exposure according to the embodiment of the present invention
  • FIGS. 9A and 9B are graphs illustrating an example of the photosensitive characteristics of the photosensitive drum 4 in the main scanning direction
  • FIG. 10 is a graph illustrating an example of the light emitting characteristics of the semiconductor laser 11 (laser diodes LD 1 to LD 4 );
  • FIG. 11 shows graphs illustrating an example of light power control under which the surface potential of the photosensitive drum 4 is corrected by changing a drive current supplied to the semiconductor laser 11 ;
  • FIG. 12 is a graph illustrating an example of the light emitting response of the semiconductor laser 11 .
  • the configuration of an image forming apparatus 1 according to the first embodiment of the present invention will be described first with reference to FIG. 1 .
  • the image forming apparatus 1 executes an image forming process of forming an image on the surface of a transfer material (printing material) P based on input image information.
  • the image information may be input from an image reading apparatus 300 or input via a network from other external apparatuses such as an information processing apparatus (computer).
  • the image forming process includes a charging process, exposure process, developing process, transfer process, and fixing process to be described hereinafter.
  • the image forming apparatus 1 sequentially executes these processes to form an image on the surface of the transfer material P.
  • the image forming apparatus 1 includes a drum-shaped photosensitive drum 4 which rotates in a direction indicated by an arrow in FIG. 1 as an example of the photosensitive member.
  • a charging roller 5 and a developing unit 6 are placed around the photosensitive drum 4 in the order named in the direction in which it rotates.
  • the charging roller 5 serves as a charging unit which charges the surface of the photosensitive drum 4 .
  • the developing unit 6 develops an electrostatic latent image formed on the surface of the photosensitive drum 4 .
  • An intermediate transfer belt 7 A and a cleaner 9 A also are placed around the photosensitive drum 4 .
  • a toner image is transferred from the photosensitive drum 4 onto the intermediate transfer belt 7 A.
  • the cleaner 9 A removes the toner remaining on the surface of the photosensitive drum 4 after the transfer of the toner image onto the intermediate transfer belt 7 A to clean this surface.
  • the intermediate transfer belt 7 A is looped around three rollers: a driving roller 7 C, a secondary transfer opposing roller 7 D, and a tension roller 7 E.
  • the tension roller 7 E maintains a constant tensile force in the intermediate transfer belt 7 A.
  • the driving roller 7 C drives the intermediate transfer belt 7 A to transport it in a direction indicated by arrows in FIG. 1 .
  • a transfer roller 8 is placed next to the intermediate transfer belt 7 A.
  • the transfer roller 8 serves to transfer a color toner image on the intermediate transfer belt 7 A onto the surface of the transfer material P.
  • the intermediate transfer belt 7 A and transfer roller 8 are placed in contact with each other in a transport path along which the transfer material P fed from a paper feed cassette (not shown) is transported (in a direction indicated by an arrow).
  • a fixing device 10 is placed downstream of the transfer roller 8 in the transport path.
  • the developing unit 6 develops an electrostatic latent image formed on the surface of the photosensitive drum 4 , using a developing material such as toner to form a developing material image (toner image) on the surface of the photosensitive drum 4 .
  • the developing unit 6 includes a plurality of developers 6 Y, 6 M, 6 C, and 6 Bk.
  • the developers 6 Y, 6 M, 6 C, and 6 Bk store developing materials of yellow (Y), magenta (M), cyan (C), and black (Bk), respectively, obtained by mixing toners and carriers at a predetermined ratio.
  • the developers develop electrostatic latent images formed on the surface of the photosensitive drum 4 , using developing materials of different colors.
  • the image forming apparatus 1 includes a main body control device 200 and image control device 3 as a control mechanism which controls the operation of the image forming apparatus 1 .
  • the image forming apparatus 1 also includes an optical scanning apparatus (exposure apparatus) 2 which emits a light beam (laser beam) to scan the surface of the photosensitive drum 4 .
  • the optical scanning apparatus 2 exposes the surface of the photosensitive drum 4 with a laser beam corresponding to input image information.
  • the main body control device 200 controls the operation of the overall image forming apparatus 1 .
  • the image control device 3 converts image information of each color into image data for laser output and outputs it to the optical scanning apparatus 2 , in accordance with an instruction from the main body control device 200 .
  • the image forming apparatus 1 Upon the start of an image forming process, the image forming apparatus 1 starts a charging process first. In the charging process, the image forming apparatus 1 applies a charging bias from a power supply (not shown) to the charging roller 5 so that the charging roller 5 uniformly charges the surface of the photosensitive drum 4 to a predetermined potential.
  • a charging bias from a power supply (not shown) to the charging roller 5 so that the charging roller 5 uniformly charges the surface of the photosensitive drum 4 to a predetermined potential.
  • the optical scanning apparatus 2 exposes the surface of the photosensitive drum 4 charged to a predetermined potential (dark portion potential).
  • the image control device 3 sequentially converts pieces of image information of Y, M, C, and Bk input from the image reading apparatus 300 into image data of these colors, and outputs them to the optical scanning apparatus 2 .
  • the optical scanning apparatus 2 irradiates the surface of the photosensitive drum 4 with laser beams L 1 based on the image data of each color to expose this surface.
  • the surface potential of a portion irradiated with the laser beams L 1 on the surface of the photosensitive drum 4 changes from the above-mentioned predetermined potential to a different potential (light portion potential).
  • an electrostatic latent image of each color is sequentially formed on the surface of the photosensitive drum 4 .
  • the electrostatic latent image of each color moves from the position at which the surface of the photosensitive drum 4 is irradiated with a laser beam by the optical scanning apparatus 2 to a developing position at which the developing unit 6 and photosensitive drum 4 contact with each other, with rotation of the photosensitive drum 4 (in a direction indicated by an arrow).
  • the developing unit 6 rotates in a direction indicated by an arrow so that a developer (the developer 6 Y, 6 M, 6 C, or 6 Bk) corresponding to the color of the electrostatic latent image to be developed moves to the developing position.
  • the developers 6 Y, 6 M, 6 C, and 6 Bk develop the electrostatic latent images formed on the surface of the photosensitive drum 4 , using toners of different colors at the developing position. More specifically, toners of different colors stored in the developers 6 Y, 6 M, 6 C, and 6 Bk adhere to the latent image portions on the surface of the photosensitive drum 4 to sequentially form a toner image of each color on the surface of the photosensitive drum 4 .
  • the formed toner image of each color moves from the developing position to a primary transfer position at which the photosensitive drum 4 and a primary transfer roller 7 B contact with each other on opposite sides of the intermediate transfer belt 7 A, with rotation of the photosensitive drum 4 .
  • the toner images of the four colors sequentially formed on the surface of the photosensitive drum 4 by the developing unit 6 sequentially undergo primary transfer on the intermediate transfer belt 7 A in superposition at a primary transfer position. That is, the M, C, and Bk toner images are transferred onto the surface of the intermediate transfer belt 7 A in superposition at the same position as that at which the Y toner image is formed on the surface of the intermediate transfer belt 7 A. As a result, a color toner image formed by the four color toner images is formed on the surface of the intermediate transfer belt 7 A.
  • the image forming apparatus 1 applies a predetermined transfer bias from a power supply (not shown) to the primary transfer roller 7 B to transfer the toner image from the photosensitive drum 4 onto the intermediate transfer belt 7 A.
  • the color toner image formed on its surface moves to a secondary transfer position at which the transfer roller 8 and secondary transfer opposing roller 7 D contact with each other on opposite sides of the intermediate transfer belt 7 A.
  • the transfer roller 8 performs secondary transfer of the toner image on the intermediate transfer belt 7 A onto the transfer material P.
  • the transfer material P having the unfixed toner image transferred on its surface is transported to the fixing device 10 along the transport path.
  • the fixing device 10 applies heat and a pressure to the transfer material P to fix the unfixed toner image on the transfer material P.
  • the cleaner 9 A removes, for example, the toner remaining on the surface of the photosensitive drum 4 after primary transfer, thereby cleaning the surface of the photosensitive drum 4 .
  • a cleaner 9 B removes, for example, the toner remaining on the surface of the intermediate transfer belt 7 A after secondary transfer, thereby cleaning the surface of the intermediate transfer belt 7 A.
  • the configuration of the optical scanning apparatus (exposure apparatus) 2 will be described next with reference to FIG. 2 .
  • the optical scanning apparatus 2 includes constituent elements other than the photosensitive drum 4 among those shown in FIG. 2 . That is, the optical scanning apparatus 2 includes a semiconductor laser 11 , laser driving device (laser control unit) 12 , collimator lens 13 , light power detection (PD) unit 14 , cylindrical lens 16 , scanner motor unit 17 , polygon mirror 17 a, f - ⁇ lens 18 , reflecting mirror 19 , and beam detection (BD) sensor 20 .
  • the semiconductor laser 11 includes a plurality of laser diodes LD and can simultaneously output a plurality of light beams (laser beams) from these plurality of laser diodes.
  • the laser driving device 12 controls driving of the semiconductor laser (its laser diodes LD) based on a drive current supplied to the semiconductor laser 11 (its laser diodes LD).
  • a laser beam output from the semiconductor laser 11 is transmitted through the collimator lens 13 , is converted into a collimated beam by the collimator lens 13 , and enters the PD unit 14 .
  • the PD unit 14 includes an internal reflecting mirror 14 a and a light power detector 14 b on its beam output surface.
  • the reflecting mirror 14 a has a property of partially reflecting a laser beam from the semiconductor laser 11 .
  • the laser beam reflected by the reflecting mirror 14 a is received by the light power detector 14 b .
  • the light power detector 14 b serves as a measuring element which measures the light power of the received laser beam.
  • the light power detector 14 b Upon receiving the laser beam, the light power detector 14 b outputs a PD current (light power detection signal) 15 corresponding to the light power (intensity) of the received laser beam to the laser driving device 12 .
  • the PD unit 14 uses the light power detector 14 b to measure the light power of a laser beam emitted by the semiconductor laser 11 , and outputs a PD current 15 corresponding to the measured light power to the laser driving device 12 .
  • the laser driving device 12 performs automatic light power control (to be described later), that is, APC (Automatic Power Control) based on the PD current 15 output from the PD unit 14 , thereby adjusting a drive current supplied to the semiconductor laser 11 .
  • APC Automatic Power Control
  • a laser beam which is output from the semiconductor laser 11 and passes through the PD unit 14 further passes through the cylindrical lens 16 and reaches the polygon mirror 17 a .
  • the polygon mirror 17 a is driven by the scanner motor unit 17 including a scanner motor, thereby rotating at a constant angular velocity. Note that the scanner motor unit 17 rotates the polygon mirror 17 a based on the control of the image control device 3 using a motor control signal 26 , as will be described later with reference to FIG. 3 .
  • the polygon mirror 17 a is a rotating polygon mirror which deflects a laser beam while rotating at a constant angular velocity so that the laser beam scans the surface of the photosensitive drum 4 .
  • the laser beam deflected by the polygon mirror 17 a enters the f- ⁇ lens 18 .
  • laser beams L 1 which travel along optical paths denoted by reference symbol L 1 in accordance with the angle of rotation of the polygon mirror 17 a scan and expose the surface of the photosensitive drum 4 to form an electrostatic latent image on the photosensitive drum 4 in one laser beam scanning cycle.
  • a laser beam L 2 which travels along an optical path denoted by reference symbol L 2 in accordance with the angle of rotation of the polygon mirror 17 a is on the edge of the laser beam scanning range, and enters the BD sensor 20 to generate a synchronization signal used in, for example, control of the image position in a direction parallel to the rotation axis of the photosensitive drum 4 and control of the rotation speed of the polygon mirror 17 a , in one laser beam scanning cycle.
  • the laser beams L 1 pass through the f- ⁇ lens 18 , are reflected by the reflecting mirror 19 , and reach the photosensitive drum 4 .
  • the f- ⁇ lens 18 has a function of performing speed conversion so that the traces of the laser beams L 1 move at a constant speed in a direction (the main scanning direction of the laser beams L 1 , that is, a direction parallel to the rotation axis of the photosensitive drum 4 ) perpendicular to the direction (the sub-scanning direction of the laser beams L 1 ) in which the photosensitive drum 4 rotates, on the surface (scanning plane) of the photosensitive drum 4 .
  • the photosensitive drum 4 is irradiated with the laser beams L 1 output from the semiconductor laser 11 to form an electrostatic latent image on the surface of the photosensitive drum 4 .
  • the laser driving device 12 supplies a drive current to the semiconductor laser 11 at a timing at which the laser beam L 2 travels along an optical path denoted by reference symbol L 2 (a timing at which the laser beam L 2 enters the BD sensor 20 ).
  • the BD sensor 20 Upon receiving the laser beam L 2 , the BD sensor 20 outputs a beam detection (BD) signal 21 as a reference to scan the photosensitive drum 4 with the laser beam L 1 .
  • the BD signal 21 output from the BD sensor 20 is supplied to the image control device 3 .
  • step S 101 the image control device 3 accepts an input of a printing command from the main body control device 200 based on an image control signal 210 .
  • step S 102 the image control device 3 outputs a rotation operation signal indicating that the polygon mirror 17 a is to be rotated to the scanner motor unit 17 in the optical scanning apparatus 2 based on a motor control signal 26 .
  • the scanner motor unit 17 starts rotation control of the polygon mirror 17 a in accordance with the rotation operation signal from the image control device 3 .
  • step S 103 the image control device 3 determines whether a motor lock signal indicating that the scanner motor which rotates the polygon mirror 17 a has entered a stable rotation state is detected, based on the motor control signal 26 output from the scanner motor unit 17 . If the image control device 3 determines in step S 103 that the motor lock signal is detected, it advances the process to step S 104 .
  • step S 104 the image control device 3 controls the laser driving device 12 based on laser control signals 23 so that the laser driving device 12 starts light emission control of the semiconductor laser 11 . The image control device 3 makes a shift to an operating mode in which BD signals 21 output from the BD sensor 20 in the optical scanning apparatus 2 are detected.
  • step S 105 the image control device 3 determines whether it is detected that BD signals 21 have been input from the BD sensor 20 in the optical scanning apparatus 2 a predetermined number of times. If the image control device 3 detects in step S 105 that BD signals 21 have been input a predetermined number of times, it advances the process to step S 106 .
  • step S 106 the image control device 3 outputs image data 22 to the laser driving device 12 in the optical scanning apparatus 2 at a timing determined with reference to that at which the BD signals 21 are detected.
  • the optical scanning apparatus 2 (laser driving device 12 and semiconductor laser 11 ) irradiates the photosensitive drum 4 with the laser beams L 1 to expose the surface of the photosensitive drum 4 with the laser beams L 1 , based on the image data 22 received from the image control device 3 , as described above. Further, the optical scanning apparatus 2 executes a process corresponding to the above-mentioned image forming process, thereby finally forming, on the transfer material P, a toner image obtained by developing an electrostatic latent image formed by the exposure of the surface of the photosensitive drum 4 .
  • step S 107 during the image forming (printing) operation, the image control device 3 determines whether a printing operation is complete. As long as the image control device 3 determines that the printing operation is incomplete, it repeats the determination process in step S 107 . However, if the image control device 3 determines that the printing operation is complete, it advances the process to step S 109 .
  • the image control device 3 controls the scanner motor unit 17 in the optical scanning apparatus 2 based on the motor control signal 26 to stop the rotation operation of the polygon mirror 17 a in step S 109 . Also, the image control device 3 controls the laser driving device 12 in the optical scanning apparatus 2 based on laser control signals 23 to turn off the semiconductor laser 11 . In this way, the image control device 3 performs stop control under which the operation of the optical scanning apparatus 2 is stopped.
  • step S 108 the image control device 3 outputs an error signal indicating that an operation error has occurred in the optical scanning apparatus 2 to the main body control device 200 based on the image control signal 210 .
  • step S 109 the image control device 3 performs stop control of the optical scanning apparatus 2 , as described above.
  • Unevenness of sensitivity may be generally present in the photosensitive drum 4 , as shown in FIG. 1 . This is because the sensitivity characteristics of the photosensitive drum 4 are uneven in all regions on the surface of the photosensitive drum 4 due to the occurrence of a variation in thickness of a photosensitive layer in a process of manufacturing the photosensitive drum 4 . Even when the entire surface of the photosensitive drum 4 having unevenness of sensitivity is charged under the same conditions, and the semiconductor laser 11 irradiates this entire surface with a laser beam in the same light power in this way, the surface potential of the photosensitive drum 4 remains uneven, so unevenness of potential (unevenness of potential characteristics) occurs.
  • FIG. 11 shows graphs illustrating an example of light power control under which the magnitude (amplitude) of a drive current supplied to the semiconductor laser 11 is changed in accordance with the sensitivity of the photosensitive drum 4 in the main scanning direction to correct the surface potential of the photosensitive drum 4 .
  • FIG. 11 shows the light emitting characteristics of the semiconductor laser 11 (the power of light from the semiconductor laser 11 ) and the latent image characteristics on the photosensitive drum 4 before and after correction of unevenness of sensitivity.
  • FIG. 11 also shows the characteristics during entire surface light emission and image (VDO) light emission. Entire surface emission corresponds herein to the case wherein the semiconductor laser 11 emits light in accordance with a drive current supplied to the semiconductor laser 11 without modulating this drive current in the image region on the surface of the photosensitive drum 4 . This means that a drive current is continuously supplied to the semiconductor laser 11 in entire surface light emission. Hence, in entire surface light emission, the power of light to which the photosensitive member is exposed by the semiconductor laser 11 depends on the magnitude (amplitude) of the drive current.
  • image light emission corresponds herein to the case wherein the semiconductor laser 11 emits light in accordance with a drive current modulated by PWM (Pulse Width Modulation) in accordance with image information in the image region on the surface of the photosensitive drum 4 .
  • PWM Pulse Width Modulation
  • a drive current having undergone pulse width modulation is continuously supplied to the semiconductor laser 11 in image light emission.
  • the power of light to which the photosensitive member is exposed by the semiconductor laser 11 depends on the magnitude (amplitude) of the drive current, and the pulse width of the drive current, which is determined based on a PWM value corresponding to image information, as will be described later.
  • the magnitude of the drive current is controlled to be constant so that the power of light from the semiconductor laser 11 stays constant.
  • the magnitude of a drive current supplied to the semiconductor laser 11 is changed so that a potential error which depends on unevenness of sensitivity (sensitivity data) of the photosensitive drum 4 is corrected in accordance with this unevenness of sensitivity.
  • the contrast voltage after the correction is maintained constant, as can be seen from FIG. 11 .
  • the contrast voltage of the photosensitive drum 4 after correction remains uneven in the main scanning direction and therefore has an error (this error will also be referred to as a “residual correction error” hereinafter), as can be seen from FIG. 11 .
  • FIG. 12 is a graph illustrating an example of the response characteristics of the semiconductor laser 11 .
  • the rated value of the power of light from the semiconductor laser 11 is 10 mW.
  • the abscissa indicates the PWM setting value (PWM value) corresponding to a drive current applied to the semiconductor laser 11
  • the ordinate indicates the average power of light when the semiconductor laser 11 emits light in accordance with the drive current having undergone PWM using each PWM setting value.
  • FIG. 12 illustrates the case wherein a quantization value of 32 levels (0 to 31) quantized using 5 bits is used as the PWM value.
  • This PWM value is used to modulate the drive current.
  • the drive current exhibits a rectangular wave having a predetermined frequency, and its waveform is shaped to have a duty ratio corresponding to the PWM value, thereby performing its PWM.
  • the PWM value increases, the time for which the drive current is supplied prolongs, so the total amount of a drive current supplied to the semiconductor laser 11 in accordance with the pulse of the drive current increases.
  • the larger the PWM value the larger the power of light from the semiconductor laser 11 also becomes, as shown in FIG. 12 .
  • FIG. 12 shows three types of response characteristics corresponding to the case wherein drive currents having different magnitudes (amplitudes) are applied to the semiconductor laser 11 , together with their theoretical values.
  • FIG. 12 shows the response characteristics when the maximum power of light P o is determined as 9.3 mW, 5 mW, and 2 mW, depending on the magnitude (amplitude) of the drive current. Note that FIG. 12 also shows an ideal power of light (ideal response characteristics for each PWM value) determined depending on the magnitude of the drive current as a theoretical value.
  • the photosensitive drum 4 is irradiated with a laser beam (main beam) from a laser light source in accordance with a drive current corresponding to image information to form a latent image corresponding to the image information on the surface of the photosensitive drum 4 .
  • the photosensitive drum 4 is irradiated with a laser beam from a laser light source in accordance with a drive current corresponding to a sensitivity correction value for the photosensitive drum 4 to reduce an error of the surface potential of the photosensitive drum 4 .
  • a first laser light source which outputs a main beam
  • a second laser light source which outputs a sub-beam
  • a drive current supplied to the first laser light source the magnitude of the drive current is maintained constant without changing it in accordance with a sensitivity correction value according to which a potential error of the photosensitive drum 4 due to its unevenness of sensitivity is reduced. That is, for each main scanning line, while the amplitude of a drive current (pulse signal) adjusted by APC is maintained constant without changing it in accordance with the sensitivity correction value, PWM based on image information is performed for the drive current. This prevents the light emitting response of a laser light source which outputs a laser beam corresponding to image information from changing with a change in magnitude of a drive current pulse.
  • the magnitude (amplitude) of the drive current is changed for each main scanning position at which the photosensitive drum 4 is irradiated with a laser beam, in accordance with a sensitivity correction value according to which unevenness of sensitivity of the photosensitive drum 4 is reduced.
  • the second light source irradiates the photosensitive drum 4 with a laser beam in light power in which a potential error of the photosensitive drum 4 due to its unevenness of sensitivity is reduced.
  • the magnitude of the drive current is changed, that is, the power of light from the laser light source is adjusted in accordance with the sensitivity correction value for the photosensitive drum 4 , no problem resulting from the factors associated with the light emitting response of the laser light source, as described above, is posed unless the adjusted power of light is relatively changed more in accordance with the PWM value.
  • the first exposure operation which employs the first laser light source and the second exposure operation which employs the second laser light source are used in combination, and the photosensitive drum 4 is irradiated with laser beams to expose the photosensitive drum 4 .
  • These laser light sources irradiate the same main scanning line (same region) on the surface of the photosensitive drum 4 with light beams in superposition.
  • the magnitude of the drive current pulse modulated by PWM is not changed, so the light emitting response of the laser light source remains the same. Hence, no error occurs in the power of light from the laser light source due to a change in light emitting response of the laser light source.
  • the magnitude of the drive current is changed in accordance with the sensitivity correction value so as to reduce unevenness of sensitivity of the photosensitive drum 4 .
  • PWM since PWM is not executed in the second exposure operation, this change in the magnitude of the drive current does not influence the light emitting response of the laser light source at all. Therefore, an error of the surface potential of the photosensitive drum 4 due to its unevenness of sensitivity, which remains due to a change in light emitting response of the laser light source, can be sufficiently reduced by executing the first and second exposure operations in combination in this embodiment, compared to a reduction in error when image light emission is performed by one-time exposure, as described above.
  • the operation of the optical scanning apparatus 2 according to this embodiment will be described in more detail below.
  • the semiconductor laser 11 is a multibeam laser which includes four laser diodes LD 1 to LD 4 corresponding to laser light sources, that is, has a four-beam configuration, as shown in FIG. 4 .
  • the semiconductor laser 11 includes the laser diodes LD 1 and LD 2 which emit main beams, and the laser diodes LD 3 and LD 4 which emit sub-beams.
  • the laser diodes LD 1 and LD 3 and the laser diodes LD 2 and LD 4 constitute different groups of laser light sources.
  • Laser light sources included in these different groups of laser light sources irradiate main scanning lines (regions) that are different between the different groups of laser light sources on the surface of the photosensitive drum 4 with laser beams in superposition.
  • the laser diodes LD 1 to LD 4 are arrayed in the semiconductor laser 11 so that laser beams from the laser diodes LD 1 and LD 3 scan the same main scanning line, and those from the laser diodes LD 2 and LD 4 scan the same main scanning line.
  • the main scanning line scanned by the laser diodes LD 1 and LD 3 and that scanned by the laser diodes LD 2 and LD 4 are different in the sub-scanning direction.
  • the semiconductor laser 11 need not always include a plurality of groups of laser light sources, and may include only one group of laser light sources. In the latter case as well, the same advantage as in this embodiment can be obtained.
  • the laser driving device 12 is connected to the image control device 3 ( FIG. 1 ) and the PD unit 14 ( FIG. 2 ) in the optical scanning apparatus 2 , and drives the laser diodes LD 1 to LD 4 provided in the semiconductor laser 11 as laser light sources, based on signals input from the image control device 3 and PD unit 14 .
  • the laser driving device 12 can control the light emitting states of the laser diodes LD 1 to LD 4 by controlling drive currents 40 a , 40 b , 41 a , and 41 b supplied (applied) to the laser diodes LD 1 to LD 4 , respectively, as will be described later.
  • the laser driving device 12 functions as an example of a control unit which controls a laser light source.
  • the laser driving device 12 includes a main current control unit 38 a (main current control unit a) and sub-current control unit 39 a (sub-current control unit b) which provide drive currents to the laser diodes LD 1 and LD 3 that emit a main beam and sub-beam, respectively, that scan the same main scanning line.
  • the main current control unit 38 a supplies a drive current to the laser diode LD 1
  • the sub-current control unit 39 a supplies a drive current to the laser diode LD 3 .
  • the laser driving device 12 also includes a main current control unit 38 b and sub-current control unit 39 b as current control units corresponding to the laser diodes LD 2 and LD 4 , respectively.
  • the main current control units 38 a and 38 b exemplify a first current providing unit
  • the sub-current control units 39 a and 39 b exemplify a second current providing unit.
  • the configurations and operations of the main current control unit 38 b and sub-current control unit 39 b are the same as those of the main current control unit 38 a and sub-current control unit 39 a , respectively, and a description thereof will not be given below as much as possible.
  • the laser driving device 12 further includes a mode control circuit 31 and PD switching circuit 36 , as shown in FIG. 4 .
  • the laser driving device 12 receives image (VDO) signals 22 a and 22 b , laser control signals 23 , a clock (CLK) signal 24 , and current control data (DATA) 25 from the image control device 3 , and receives a PD current 15 from the PD unit 14 .
  • the laser control signals 23 include a light power control signal 23 a and two channel selection signals 23 b and 23 c , and are input to the mode control circuit 31 in the laser driving device 12 , as shown in FIG. 5 .
  • the image control device 3 controls the operating state (operating mode) of the laser driving device 12 based on a combination of signal values of the signals included in the laser control signals 23 . That is, the mode control circuit 31 determines the operating mode to be used in the laser driving device 12 , in accordance with a combination of signal values of the signals 23 a to 23 c included in the laser control signals 23 .
  • operating modes that can be selected by the mode control circuit 31 include, for example, light power control (APC) modes (LDAm_APC, LDBm_APC, LDAs_APC, and LDBs_APC) for the laser diodes LD 1 to LD 4 , respectively, a forcible turn-off mode (OFF), and an image light emission mode (VDO).
  • FIG. 5 shows the operation of the laser driving device 12 in about one cycle in which a laser beam from the semiconductor laser 11 performs main scanning on the surface of the photosensitive drum 4 . Note that one main scanning cycle corresponds to a cycle in which BD signals 21 are detected.
  • the APC modes for the laser diodes LD 1 to LD 4 and the forcible turn-off mode are used in a non-image formation duration, in which no image is formed, of one scanning cycle in which the laser beam scans the photosensitive drum 4 , as shown in FIG. 5 .
  • the image light emission mode is used in an image formation duration, in which an image is formed, of one scanning cycle in which the laser beam scans the photosensitive drum 4 .
  • the mode control circuit 31 controls the main current control units 38 a and 38 b and sub-current control units 39 a and 39 b based on main sampling signals (MSH) 33 , sub-sampling signals (SSH) 34 , and forcible turn-off signals (FOFF) 35 so that the laser driving device 12 operates in the determined operating mode.
  • MSH main sampling signals
  • SSH sub-sampling signals
  • FOFF forcible turn-off signals
  • the PD unit 14 outputs, to the laser driving device 12 , a PD current 15 corresponding to the intensity (light emitting intensity) of a laser beam emitted by each of the laser diodes LD 1 to LD 4 in the semiconductor laser 11 .
  • the PD current 15 input to the laser driving device 12 is supplied to the PD switching circuit 36 .
  • the PD switching circuit 36 switches the destination, to which the PD current 15 input from the PD unit 14 is supplied, between the main current control units 38 a and 38 b and sub-current control units 39 a and 39 b , in accordance with the output mode determined under the control of the mode control circuit 31 .
  • the PD switching circuit 36 determines an output mode in accordance with PD switching signals 32 input from the mode control circuit 31 .
  • the mode control circuit 31 changes the PD switching signals 32 , that is, PD switching signals 32 a and 32 b output to the PD switching circuit 36 in accordance with the operating mode determined in the foregoing way, as shown in FIG. 5 .
  • the output mode (reference numeral 502 in FIG. 5 ) corresponding to the destination to which the PD current 15 is supplied from the PD switching circuit 36 is switched in response to changes in PD switching signals 32 a and 32 b sent from the mode control circuit 31 to the PD switching circuit 36 .
  • the PD switching circuit 36 sets the destination to which the PD current 15 is supplied from the PD switching circuit 36 to one of the four current control units. In this way, the PD switching circuit 36 determines an output mode in accordance with a combination of signal values of the PD switching signals 32 a and 32 b from the mode control circuit 31 .
  • main current control units 38 a and 38 b and sub-current control units 39 a and 39 b will be described below with reference to a timing chart shown in FIG. 5 , together with a description of the operation of the laser driving device 12 in each operating mode (reference numeral 501 in FIG. 5 ).
  • a drive current supplied from the main current control unit 38 a to the laser diode LD 1 is controlled so that the power of light from the laser diode LD 1 is controlled to that determined in advance.
  • LLBm_APC APC mode for the laser diode LD 2
  • main current control unit 38 b main current control unit 38 b for the laser diode LD 2 will not be given.
  • the mode control circuit 31 Upon selecting the APC mode for the laser diode LD 1 , the mode control circuit 31 operates in the following way.
  • the mode control circuit 31 sets a sample/hold (S&H) circuit 49 in the main current control unit 38 a in a sample (enabled) state based on a main sampling signal 33 a .
  • the mode control circuit 31 controls a VDO control circuit 54 based on a forcible turn-off signal 35 a to forcibly sets, in an ON (light emitting) state, the laser diode LD 1 to which a drive current is supplied from the main current control unit 38 a .
  • the VDO control circuit 54 forcibly turns on a transistor 55 shown in FIG. 4 so that the laser diode LD 1 emits light.
  • the mode control circuit 31 controls the PD switching circuit 36 based on a PD switching signal 32 to switch, to the main current control unit 38 a , the destination (reference numeral 502 in FIG. 5 ) to which the PD current 15 is supplied from the PD switching circuit 36 .
  • the PD current 15 input from the PD unit 14 starts to be supplied to the main current control unit 38 a as a PD current PDam.
  • the PD unit 14 outputs a PD current 15 corresponding to the intensity (light emitting intensity) of a laser beam (main beam) emitted by the laser diode LD 1 .
  • the PD current 15 that is, the PD current PDam input to the main current control unit 38 a via the PD switching circuit 36 is converted into a voltage by a variable resistor 48 . The converted voltage is applied to a comparator 46 .
  • the comparator 46 compares the applied voltage with a reference voltage which is applied from a reference voltage generator 47 and corresponds to power of light (light power control value P mtgt ) determined in advance.
  • the comparator 46 outputs, as the comparison result, a signal corresponding to the difference between those voltages to the sample/hold circuit 49 in a sample (enabled) state, thereby changing the voltage of a hold capacitor 50 .
  • the hold capacitor 50 is charged at a time constant unique to itself.
  • the voltage of the hold capacitor 50 is input to the input terminal of a current driver 51 .
  • the current driver 51 operates so that a current corresponding to a driving resistance 52 and the voltage of the hold capacitor 50 is output from a mirror circuit 53 to the laser diode LD 1 .
  • one transistor (on the input side) in the mirror circuit 53 is connected to the current driver 51 , while the other transistor (on the output side) is connected to the transistor 55 .
  • this current is supplied to the transistor 55 . Since the transistor 55 is forcibly turned on by the VDO control circuit 54 , the current from the mirror circuit 53 is supplied to the laser diode LD 1 as the drive current 40 a , that is, a drive current LDma.
  • the laser diode LD 1 When the laser diode LD 1 is supplied with the drive current 40 a , it emits light in power corresponding to this drive current, and outputs a laser beam.
  • the laser beam output from the laser diode LD 1 is detected by the PD unit 14 , and a PD current 15 corresponding to the intensity of the laser beam is input to the main current control unit 38 a as the PD current PDam.
  • the comparator 46 compares the voltage obtained by converting the PD current 15 with the reference voltage, as described above.
  • the main current control unit 38 a controls a drive current supplied to the laser diode LD 1 so that the voltage obtained by converting the PD current 15 comes close to the reference voltage, in accordance with the comparison result. In this way, the laser driving device 12 controls the light power of the laser diode LD 1 to light power P mtgt determined in advance.
  • the mode control circuit 31 selects a forcible turn-off mode (OFF), as shown in FIG. 5 .
  • the mode control circuit 31 operates in the following way.
  • the mode control circuit 31 controls the VDO control circuit 54 based on a forcible turn-off signal 35 a to forcibly set the laser diode LD 1 in an OFF state. More specifically, the mode control circuit 31 turns off the forcible turn-off signal 35 a to control the VDO control circuit 54 so as to switch the state of the transistor 55 to an OFF state. This cuts off the provision of the drive current 40 a from the mirror circuit 53 to the laser diode LD 1 , so the laser diode LD 1 changes to an OFF state.
  • the mode control circuit 31 sets the sample/hold circuit 49 in a hold (disabled) state based on the main sampling signal 33 a . This keeps inputting the voltage of the charged hold capacitor 50 to the current driver 51 .
  • the laser diode LD 1 emits light in power adjusted by APC of (1) in an image light emission mode (VDO) (to be described later).
  • the mode control circuit 31 Upon selecting the APC mode for the laser diode LD 3 , the mode control circuit 31 operates in the following way.
  • the mode control circuit 31 sets a sample/hold (S&H) circuit 59 in the sub-current control unit 39 a in a sample (enabled) state based on a sub-sampling signal 34 a , and switches a switching circuit 61 so that the sample/hold circuit 59 and the current driver 51 are connected to each other. Also, the transistor 55 is forcibly turned on in accordance with the sub-sampling signal 34 a . Note that as can be seen from FIG.
  • the sub-current control unit 39 a executes an operation equivalent to that of APC by the main current control unit 38 a described above.
  • a drive current applied from the laser driving device 12 to the laser diode LD 3 is determined so that the power of light from the laser diode LD 3 is equal to power of light P mtgt determined in advance, as shown in FIG. 5 .
  • VDO Image Light Emission Mode
  • the sample/hold circuit 49 of the main current control unit 38 a and the sample/hold circuit 59 of the sub-current control unit 39 a change to a hold (disabled) state in accordance with the main sampling signal 33 a and sub-sampling signal 34 a , respectively.
  • the VDO control circuit 54 outputs, to the transistor 55 , a VDO signal 22 a corresponding to image information supplied from the image control device 3 .
  • the sub-sampling signal 34 a switches the state of the switching circuit 61 to that in which the sample/hold circuit 59 and a digital-to-analog conversion circuit (DAC) 66 are connected to each other.
  • DAC digital-to-analog conversion circuit
  • the current driver 51 of the main current control unit 38 a supplies, to the mirror circuit 53 , a current which is determined by APC and corresponds to the driving resistance 52 and the voltage of the hold capacitor 50 .
  • the mirror circuit 53 changes the current input from a current driver 62 at a predetermined mirror ratio, and supplies it to the laser diode LD 1 as a drive current LDma.
  • This drive current is supplied from the mirror circuit 53 to the laser diode LD 1 via the transistor 55 .
  • the transistor 55 switches the drive current supplied from the mirror circuit 53 to the laser diode LD 1 , in accordance with a VDO signal 22 a supplied via the VDO control circuit 54 .
  • the magnitude of the drive current is constant at a current level determined by APC.
  • the drive current modulated by PWM is supplied to the laser diode LD 1 so that the laser diode LD 1 irradiates the photosensitive drum 4 with a laser beam (main beam) corresponding to image information.
  • the voltage of a hold capacitor 60 determined by APC is applied to the DAC 66 via the switching circuit 61 as a reference voltage.
  • the reference voltage corresponds to power of light of 100% in FIG. 12 .
  • the DAC 66 receives the clock (CLK) signal 24 and the current control data (DATA) 25 from the image control device 3 .
  • the DAC 66 converts the current control data 25 from a digital signal into an analog signal and outputs it in accordance with the timing of the input clock signal 24 .
  • the current control data 25 corresponds herein to a correction value used to reduce unevenness of potential characteristics for each region on the surface of the photosensitive drum 4 .
  • the analog signal which is output from the DAC 66 and corresponds to the correction value is input to the current driver 62 via a lowpass filter (LPF) 67 .
  • the current driver 62 supplies a current corresponding to a driving resistance 63 and the input from the lowpass filter 67 to a mirror circuit 64 .
  • the mirror circuit 64 changes the current input from the current driver 62 at a predetermined mirror ratio, and supplies it to the laser diode LD 3 as a drive current LDsa.
  • This drive current is supplied to the laser diode LD 3 via a transistor 65 turned on in accordance with the sub-sampling signal 34 a .
  • the drive current supplied to the laser diode LD 3 changes in accordance with the current control data 25 (sensitivity correction value) so that the laser diode LD 3 irradiates the photosensitive drum 4 with a laser beam (sub-beam) in light power corresponding to the sensitivity correction value, as shown in FIG. 5 .
  • laser beams (main beam and sub-beam) output from the laser diode LD 1 (first laser light source) and the laser diode LD 3 (second laser light source) onto the photosensitive drum 4 scan the same main scanning line on the surface of the photosensitive drum 4 , as will be described later.
  • This makes it possible to expose the photosensitive drum 4 without changing a drive current supplied to the laser diode LD 1 which outputs a laser beam corresponding to image information.
  • This also makes it possible to sufficiently reduce an error of the surface potential of the photosensitive drum 4 due to its unevenness of sensitivity using a laser beam emitted by the laser diode LD 3 .
  • the operations of the laser diodes LD 2 and LD 4 and the main current control unit 38 b and sub-current control unit 39 b corresponding to them are the same as those of the laser diodes LD 1 and LD 3 and the main current control unit 38 a and sub-current control unit 39 a , respectively, and a description thereof will not be given.
  • a scanning method corresponding to the exposure scheme using main beams and sub-beams output from the laser diodes LD 1 to LD 4 of the semiconductor laser 11 will be described next with reference to FIGS. 6A , 6 B, 7 , and 8 .
  • a contrast voltage is generated between different potentials on the surface of the photosensitive drum 4 depending on the difference in exposure scheme. More specifically, a contrast voltage V cont is generated between potentials different between background exposure and image exposure. In developing the formed electrostatic latent image, toner in an amount corresponding to the contrast voltage V cont adheres to the surface of the photosensitive drum 4 . To prevent the occurrence of unevenness of density in the developed image, it is necessary to maintain the contrast voltage V cont constant.
  • the contrast voltage V cont corresponds to the potential difference between a developing bias V DC and a dark portion potential V D corresponding to the surface potential of the portion on the surface of the photosensitive drum 4 , which is not irradiated with laser beams from the laser diodes LD 1 and LD 3 .
  • the contrast voltage V cont corresponds to the potential difference between a developing bias V DC and a light portion voltage V L corresponding to the surface potential of the portion on the surface of the photosensitive drum 4 , which is irradiated with laser beams from the laser diodes LD 1 and LD 3 .
  • the type of beam to be output onto the photosensitive drum 4 first is selected between a main beam and a sub-beam for each main scanning line, in accordance with whether the image forming apparatus 1 executes background exposure or image exposure. That is, the type of exposure operation to be executed first is selected between a first exposure operation which uses a main beam and a second exposure operation which uses a sub-beam, depending on the exposure scheme. This results from the above-mentioned difference in contrast voltage V cont .
  • FIGS. 6A and 6B are views showing the orders of use of main beams and sub-beams which irradiate each main scanning line with in the respective exposure schemes: background exposure and image exposure. FIGS.
  • FIGS. 7 and 8 are graphs showing the light emitting characteristics of the semiconductor laser 11 and the latent image characteristics on the photosensitive drum 4 in the first and second exposure operations using different laser light sources for the same main scanning line. Note that FIG. 7 shows the case of background exposure, and FIG. 8 shows the case of image exposure.
  • the laser diodes LD 1 and LD 3 included in the same group of laser light sources irradiate the same main scanning line with a main beam and a sub-beam, respectively, as shown in FIGS. 6A and 6B .
  • the laser diodes LD 2 and LD 4 included in the same group of laser light sources irradiate a main scanning line, different from that irradiated by the laser diodes LD 1 and LD 3 included in the group of laser light sources different from that including the laser diodes LD 2 and LD 4 , with a main beam and a sub-beam, respectively.
  • the order of use of a main beam and a sub beam which scan each main scanning line is different depending on the exposure scheme, as shown in FIGS. 6A and 6B .
  • the laser diode LD 3 or LD 4 irradiates this irradiated main scanning line with a sub-beam in superposition, as shown in FIG. 6A .
  • a fluctuation (error) corresponding to the main scanning position occurs in the surface potential V D and V L , depending on unevenness of sensitivity of the photosensitive drum 4 , as shown in FIG. 7 .
  • a contrast voltage V cont is generated between a developing bias V DC and a dark portion potential V D .
  • the potential error that has occurred in the dark portion potential V D after the first exposure operation which uses a main beam need only be reduced by the second exposure operation which uses a sub-beam.
  • the first exposure operation is executed using a main beam
  • the second exposure operation is executed using a sub-beam.
  • image light emission is performed by the first exposure operation
  • the light emitting response of the laser light source used changes.
  • An error of the surface potential of the photosensitive drum 4 thus cannot be sufficiently corrected and remains, as described above.
  • the first exposure operation does not change the light emitting response of the laser light source used, and the second exposure operation is not influenced by the light emitting response of this laser light source.
  • exposing each main scanning line twice by the first and second exposure operations makes it possible to reduce an error of the surface potential of the photosensitive drum 4 due to its unevenness of sensitivity (potential characteristics) without allowing this error to remain due to a change in light emitting response of the laser light source used. Also, the following image exposure is done by executing the first and second exposure operations in the order reverse to that in the background exposure, but produces the same advantage as that produced by the background exposure.
  • the laser diode LD 1 or LD 2 irradiates this irradiated main scanning line with a main beam in superposition, as shown in FIG. 6B .
  • a contrast voltage V cont is generated between a developing bias V DC and a light portion voltage V L .
  • the order of use of a main beam and a sub-beam which scan each main scanning line on the surface of the photosensitive drum 4 is set in accordance with whether the exposure scheme is background exposure or image exposure.
  • the contrast voltage V cont after exposure stays constant. That is, regardless of the exposure scheme, a potential error of the photosensitive drum 4 due to its unevenness of sensitivity can be reduced, thus suppressing degradation in image quality.
  • a sensitivity correction value that is, current control data (DATA) 25 to be input to the sub-current control units 39 a and 39 b is generated by the image control device 3 .
  • the image control device 3 can calculate a sensitivity correction value by an arithmetic operation to be described below, based on sensitivity data measured in advance using the photosensitive drum 4 provided in the image forming apparatus 1 .
  • This sensitivity data can be stored in a nonvolatile storage device such as an EEPROM provided in the laser driving device 12 or photosensitive drum 4 .
  • this storage device is provided in, for example, the photosensitive drum 4
  • the image control device 3 need only read out the sensitivity data from this storage device and newly calculate a sensitivity correction value based on the readout sensitivity data every time replacement of the photosensitive drum 4 is detected.
  • the image control device 3 holds the calculated sensitivity correction value in the internal storage device, and outputs the sensitivity correction value held in the storage device to the laser driving device 12 as the current control data (DATA) 25 during its operation in the above-mentioned image light emission mode.
  • the sensitivity correction value input to the laser driving device 12 is input to the sub-current control units 39 a and 39 b.
  • FIG. 9A is a graph illustrating an example of the sensitivity characteristics of the photosensitive drum 4 in the main scanning direction on the photosensitive drum 4 .
  • FIG. 9A shows a main scanning position d on the abscissa, and a sensitivity ⁇ (d) of the photosensitive drum 4 at each of 13 positions in the main scanning direction on the ordinate. More specifically, assuming that the correction range in the main scanning direction (the width in the main scanning direction, across which an electrostatic latent image is formed) on the photosensitive drum 4 is 300 mm (the main scanning position d falls within the range of ⁇ 150 mm to +150 mm), FIG. 9A shows the sensitivity ⁇ (d) of the photosensitive drum 4 at each of 13 positions in this range.
  • the photosensitive drum 4 has a sensitivity ⁇ (d) which varies in each individual main scanning position d, that is, unevenness of sensitivity (a difference in sensitivity). This unevenness of sensitivity results from unevenness of thickness of a photosensitive layer generated upon the manufacture of the photosensitive drum 4 .
  • the sensitivity ⁇ (d) has a value measured using the photosensitive drum 4 upon, for example, the manufacture of the photosensitive drum 4 , and is stored in the storage device of the laser driving device 12 or photosensitive drum 4 as sensitivity data, as described above.
  • the image control device 3 calculates a sensitivity correction value using sensitivity data as shown in FIG. 9A .
  • the procedure of calculating a sensitivity correction value will be described below with reference to FIGS. 9B and 10 by taking the sensitivity data shown in FIG. 9A as an example.
  • FIG. 9B shows the correction value corresponding to each main scanning position d shown in FIG. 9A in the main scanning direction on the photosensitive drum 4 .
  • the sensitivity correction value has a resolution of 8 bits (256 levels)
  • the image control device 3 calculates the ratio of the sensitivity ⁇ (d) and calculates a correction value ⁇ based on the calculated ratio.
  • the image control device 3 similarly calculates a correction value ⁇ at each main scanning position d in the main scanning direction.
  • FIG. 10 is a graph illustrating an example of the light emitting characteristics of the semiconductor laser 11 (laser diodes LD 1 to LD 4 ), and shows the drive current I versus light power P characteristics.
  • the above-mentioned arithmetic operation is done for all main scanning positions d determined in advance to calculate sensitivity correction values at all these main scanning positions d.
  • the image control device 3 holds the calculated sensitivity correction values in the storage device, and outputs them to the laser driving device 12 as the current control data (DATA) 25 in the image light emission mode.
  • the optical scanning apparatus (exposure apparatus) 2 employs both a first laser light source (first light source) which outputs a laser beam (light beam) corresponding to a drive current corresponding to image information, and a second laser light source (second light source), in order to irradiate the surface of the photosensitive drum (photosensitive member) 4 with a plurality of laser beams.
  • the second laser light source irradiates the photosensitive drum 4 with a laser beam in accordance with a drive current corresponding to a correction value according to which unevenness of potential characteristics of the surface of the photosensitive drum 4 due to its unevenness of sensitivity is reduced.
  • the same main scanning line (the same region) on the surface of the photosensitive drum 4 is irradiated with laser beams which are output from the first and second laser light sources onto the photosensitive drum 4 . That is, on the surface of the photosensitive drum 4 , a main scanning line (region) irradiated with a laser beam from one of the first and second laser light sources is irradiated with a laser beam from the other in superposition. It can be determined whether the first or second laser light source irradiates each main scanning line with a laser beam first, in accordance with the exposure scheme of the image forming apparatus 1 equipped with the optical scanning apparatus 2 .
  • the second laser light source can irradiate the photosensitive drum 4 with a laser beam for reducing a charged potential error (unevenness of potential characteristics) of the photosensitive drum 4 due to its unevenness of sensitivity. This makes it possible to avoid an insufficient reduction in charged potential error using a laser beam corresponding to a sensitivity correction value due to a change in light emitting response of the laser light source corresponding to a change in drive current, and to reduce this potential error.
  • the present invention is also applicable to the use of only one group of laser light sources. In the latter case as well, the same advantage as in this embodiment can be obtained. In this case, none of the laser diodes LD 2 and LD 4 and the main current control unit 38 b and sub-current control unit 39 b corresponding to them are necessary.
  • the present invention is applicable not only to the above-mentioned embodiment in which the first and second exposure operations are executed in each region on the surface of the photosensitive drum 4 by a plurality of laser light sources, but also to an embodiment in which the first and second exposure operations are executed in each region on the surface of the photosensitive drum 4 by a single laser light source.
  • the first and second exposure operations executed by the first and second laser light sources, respectively, as described above can be executed on each main scanning line (each region) by one laser light source in time series, that is, twice.
  • the first and second exposure operations can be executed for each main scanning line using light beams in superposition, as in the above-mentioned embodiment.
  • the first and second exposure operations may be executed by each of a plurality of laser light sources which scan different main scanning lines in the sub-scanning direction. In this case, it is possible to reduce an error of the surface potential of the photosensitive drum 4 while improving the exposure rate.

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